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Old 10-04-2018, 11:07   #91
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Re: Thorny subject: Sextant and GPS era

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Originally Posted by Davidhoy View Post
Quote from the Wikipedia page on "Selective Availability" - https://en.wikipedia.org/wiki/Error_...e_availability


On 19 September 2007, the United States Department of Defense announced that future GPS III satellites will not be capable of implementing SA,[12] eventually making the policy permanent.[13]


And from the official government GPS website, https://www.gps.gov/systems/gps/modernization/sa/


Selective Availability (SA) was an intentional degradation of public GPS signals implemented for national security reasons.



In May 2000, at the direction of President Bill Clinton, the U.S government discontinued its use of Selective Availability in order to make GPS more responsive to civil and commercial users worldwide.



The United States has no intent to ever use Selective Availability again.

In September 2007, the U.S. government announced its decision to procure the future generation of GPS satellites, known as GPS III, without the SA feature. Doing this will make the policy decision of 2000 permanent and eliminate a source of uncertainty in GPS performance that had been of concern to civil GPS users worldwide.



Well that’s both good and bad.

The bad part is that if the DoD thinks the GPS system is being used in an attack on the US they are going to completely turn off the civilian part of the system. The military side of the system will still operate but the signals are “encrypted” so you and I won’t be able to use them.

Makes sense, there’s no point in having SA when the WAAS, DGPS and LAAS systems correct to better than plain GPS. So why spend the money on that capability.
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Old 10-04-2018, 11:11   #92
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Re: Thorny subject: Sextant and GPS era

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Any lightning strike that fries you backup electronics in the ammo case is probably going to dry all the quartz watches too.

The mechanical watch is just the ultimate backup timepiece. I got a Molinja for $35 off eBay.
Why bother with an ammo box then?
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Old 10-04-2018, 11:12   #93
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Re: Thorny subject: Sextant and GPS era

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Magnetic compasses don’t work in subs, the hull completely surrounds the compass and prevents the compass from giving an accurate reading.

Subs use gyrocompasses which are complex electro-mechanical devices that have a different set of problems to deal with.
gyrocompasses were used by aircraft, still are. But in that case they require engine vacuum to run them. Of course when they lose the engine ... (edit: more modern are electric, but training Cessna and the like are vacuum)

Of course subs also share this issue, thats why the repeater. Kind of the same effect one gets in a steel yacht, which also use repeaters. But subs carry much more equipment to keep themselves informed of their surrounds when submerged.
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Old 10-04-2018, 11:23   #94
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Re: Thorny subject: Sextant and GPS era

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We need precise position data for PILOTAGE, not so much for navigation.

I would not freak out in the middle of the Atlantic if I lost all position data.

I would just do dead reckoning, keep a good hourly log of speed and course, and you'll at least hit the right piece of land. When you get in sight of land, then you just do normal non-electronic pilotage. You can get a very precise position in pilotage waters with a normal three point fix.
What size piece of land?

It seems to me that a novice with a plastic sextant and a couple hours practice can verify position within a couple miles of latitude and a couple dozen miles of longitude using series of noon sun sights and a somewhat accurate watch. Dead reckoning from there would give confidence in finding Bermuda/Kiribati after a major storm/stuck in the stream/lighting strike/wind shift after the front passes while exhausted crew fell asleep on watch/etc.

With sufficient water, your average lost sailor could continue on and hit Portugal/New Guinea by DR alone. But a smallish lowish landmass when something big has probably gone wrong?
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Old 10-04-2018, 11:31   #95
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Re: Thorny subject: Sextant and GPS era

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What size piece of land?
But a smallish lowish landmass when something big has probably gone wrong?
you can get lucky

most times Pacific islands are cloud magnets
if there are lots of clouds look for the ones with green undersides, reflecting the colour of the island. You can see these quite some distance off because of their height

but as I say, lucky ..
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Old 10-04-2018, 11:34   #96
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Re: Thorny subject: Sextant and GPS era

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Why bother with an ammo box then?
A METAL box provides some protection against the electrical fields that surround the strike. So the stuff in the box may survive a strike that wipes out all the fixed mount equipment outside the box. A steel box will give additional protection against the magnetic fields too.

If the strike is close enough even the ammo box may not protect the electronics. Then you have to go all mechanical, hence the mechanical watch.

Look up Faraday cage and lightning strike for further confusing reading.
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Old 10-04-2018, 11:39   #97
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Re: Thorny subject: Sextant and GPS era

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Why bother with an ammo box then?
Because it usually works.
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Old 10-04-2018, 11:44   #98
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Re: Thorny subject: Sextant and GPS era

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Originally Posted by Flagman101 View Post
Why bother with an ammo box then?
maybe this helps

https://en.wikipedia.org/wiki/Faraday_cage
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Old 10-04-2018, 11:47   #99
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Re: Thorny subject: Sextant and GPS era

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Dont worryI am quite familiar with the faraday principal. My first job was as a Linemen.
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Old 10-04-2018, 11:53   #100
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Re: Thorny subject: Sextant and GPS era

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Well that’s both good and bad.

The bad part is that if the DoD thinks the GPS system is being used in an attack on the US they are going to completely turn off the civilian part of the system. The military side of the system will still operate but the signals are “encrypted” so you and I won’t be able to use them.

Makes sense, there’s no point in having SA when the WAAS, DGPS and LAAS systems correct to better than plain GPS. So why spend the money on that capability.
SA is/was the civilian part of GPS. The military's only choice in the event of a future conflict will be to turn the entire GPS system off. Extremely unlikely, as their own weapons systems heavily rely on GPS. But, if the proverbial crap hits the fan, and the military needs to turn off GPS, I personally would like to be "lost" somewhere very far from any hostilities.
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Old 10-04-2018, 11:58   #101
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Re: Thorny subject: Sextant and GPS era

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Originally Posted by Lrak View Post
What size piece of land?

It seems to me that a novice with a plastic sextant and a couple hours practice can verify position within a couple miles of latitude and a couple dozen miles of longitude using series of noon sun sights and a somewhat accurate watch. Dead reckoning from there would give confidence in finding Bermuda/Kiribati after a major storm/stuck in the stream/lighting strike/wind shift after the front passes while exhausted crew fell asleep on watch/etc.

With sufficient water, your average lost sailor could continue on and hit Portugal/New Guinea by DR alone. But a smallish lowish landmass when something big has probably gone wrong?
Fair enough.

Note that I never said that celestial is useless. Not by any means! I just said that in most cases it would not be a disaster to fall back on DR. If I'm trying to find Ireland crossing W to E, I would be quite OK with DR, for example.

And as Zulu40 said, there are ways to find islands without position data. Columbus found the Azores, after all.
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Old 10-04-2018, 12:36   #102
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Re: Thorny subject: Sextant and GPS era

After 100 replies.
If you leave out commercial and military boats that have very little in commun with our little boats.
No pleasure sailboat actually went lost at sea because of GPS failure and not having a sextant.

Yes GPS can and do fail. But sextant is not your only hope if you happen to not have one or know how to use it.
DR, proper navigational skill will always get you to land.
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Old 10-04-2018, 12:41   #103
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Re: Thorny subject: Sextant and GPS era

I read a very funny story some time ago by a young woman who set out from sydney with her Dad in a fishing boat – he wanted to go for a look at Lord Howe Island as an adventure. This was in the pre-gps era. Long story short; they had charts, I dont recall if they had a sextant/almanac but I dont think so – which is why they spent a week flubbing around somewhere 500 miles off the Queensland coast without ever finding Lord Howe – when they eventually turned west and found the coast they had to ask a local fisherman where they were before getting their bearings to head back to sydney.
As you were asking about actual blue water experience; my father was the primary navigator when he and 4 mates used to do the Auckland-Noumea and Auckland-Suva races in the 70's – I remember him telling me about the anxiety he felt because a 10 mile error was not unusual and in the wrong circumstances could send you right past a small pacific island without sighting land (he was always proud of the fact that their target came up dead ahead on every trip he navigated). He taught me traditional navigation and I have a healthy respect for the vagueries thereof, as well as an appreciation of the advantages of gps tech.
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Old 10-04-2018, 12:45   #104
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Re: Thorny subject: Sextant and GPS era

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After 100 replies.
If you leave out commercial and military boats that have very little in commun with our little boats.
No pleasure sailboat actually went lost at sea because of GPS failure and not having a sextant.

Yes GPS can and do fail. But sextant is not your only hope if you happen to not have one or know how to use it.
DR, proper navigational skill will always get you to land.
absolutely
a good working knowledge of how things used to be done, right back to Captain Cook and the Longitude story isnt going to hurt though

after all, youre the Captain, you are supposed to know ****
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Old 10-04-2018, 12:59   #105
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Re: Thorny subject: Sextant and GPS era

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No experience losing GPS etc., but perhaps an elementary comment for those not familiar with celestial navigation who may read this thread. Celestial is not like GPS navigation as it is not self-contained. You don't pick-up a sextant and do some arcane magic to tell your position. In basic terms, you need to know how to take a precise angular measurement of a specific body, where that body is above the earth at that time, and how to find your distance from that earthly position. Getting that angular measurement is only one-fifth of the problem.

Interesting to note that the US Naval Academy does not teach CELNAV any more, the Brits do, they are more traditional. Warships carry numerous GPS systems so less chance of them losing a position.
I carry a German Frieberger; however, my cruising days are over if anyone is interested in purchasing.
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